A field emission display device typically includes a pair of substrates which are maintained in a spaced apart, yet parallel relationship with one another. A plurality of cathodes and phosphors are disposed in a predetermined pattern upon the inner surface of one of the substrates.
In practice, electron emission is realized by Schottky effect generated by providing very high electric field between cathode and anode. Such emitted electrons strike phosphors to excite light from the display device.
The field emission display device is classified into two types. One is a diode type having an anode and a cathode, and the other is a triode type having a grid disposed between the anode and cathode. The field emission display is appropriate to apply in a large sized display and has an advantage of lowering electric power consumption. The contrast and brightness of the field emission display depend on the amount of electrons emitted from the cathode. In order to assure the large amount of electron emission, the cathode is designed to have an emitting surface area as large as possible by providing prominence and depressions.
To form the cathode on a supporting substrate, a metallic thin layer with high melting point selected from the group consisting of tungsten and molybdenum is first applied on the supporting substrate and is then etched by a laser abrasion process to have a sharp point tip. However, since this process requires a highly accurate exposure and etching technique, it is not appropriate to apply this process in making a display having a large size screen.
That is, as shown in FIG. 3, a cathode electrode 24 having a sharp point tip is formed on a supporting substrate 20 to be enclosed by an insulating layer 22. The cathode electrode is disposed opposing a phosphor 32 applied on an anode electrode 30 formed on a front substrate 28. A grid 34 is formed on the insulating layer 22 to control electrons emitted from the cathode electrode 24.
In the above described convention field emission display device, since the sharp point tip is easily damaged from a shock generated when an arc is generated, the life span of the display is reduced. The sharp point tip requires high operating voltage to emit electron.
In addition, since it is very difficult to maintain a distance between the inner circumference of the grid 34 and a top sharp point of the cathode electrode, luminance difference may occurs on a screen.
In addition, U.S. Pat. No. 5,430,348 to Robert C. Kane discloses a field emission display comprising a diamond cathode coated with an inversion layer. U.S. Pat. Nos. 5,548,185 and 5,601,966 to Nalin Kumar disclose a field emission display device using an amorphic diamond film.
U.S. Pat. No. 5,382,867 to Maruo discloses a field emission display device comprising a cathode having a sawtooth-shaped surface which allows the display device to operate at low voltage. However, his still requires a high accurate etching process.
Generally, the diamond is well known as the most stable material, the principal ingredient of which is carbon.
As shown in FIG. 4, the diamond has a tetragonal crystal structure having hexagon (111) surface.
A disconnected end portion of the diamond is used as a passage for emitting electron. That is, when doping boron or nitrogen on the surfaces (111), since negative electron affinity phenomenon occurs, energy level of a conduction band becomes higher than that of a free electron, allowing a self-electron emission and low-voltage operation.
However, to make the cathode using diamond, the highly precise etching process is still required and increase the manufacturing costs.
Therefore, instead of the diamond, a material, a principal ingredient of which is graphite has been considered in the present invention.
As shown in FIG. 5, the graphite has a crystal structure similar to that of the diamond. That is, the crystal structure of the graphite is comprised of a plurality of hexagon surfaces (0001) which is similar to those of diamond (111) surface. However, the surfaces (0001) have a powerful double bond structure but a weak vanderwaals bond between surfaces, so have a strong anisotropy characteristics. The thermal and electric conductibilities are good on the surfaces (0001) but not in a vertical direction of the surfaces (0001). Since the coupling state between the surfaces (0001) is weak, the structure is easily broken.
However, because the corners of the surfaces (0001) are in an intensive covalent bond state, the corners can be used as an electron emission tip. Furthermore, when the graphite is broken by outer force, the broken surface provides a newly formed surfaces (0001), maintaining the electron emission quality. In addition, since the graphite inherently includes nitrogen impurities, the negative electron affinity can be generated without going through a specific process such that a low voltage operation can be expected.
However, in case of a triode field emission display device, the difficulty in constantly maintaining a distance between the inner circumference of the grid and a top sharp point of the cathode electrode still remains.